1. Field of the Invention
The invention relates generally to electronic ballast systems and, more particularly, the invention relates to an electronic ballast system that controls the duty cycle of a pulse-width modulated lamp drive signal based on a lamp current.
2. Description of Related Technology
Generally speaking, electronic ballast systems initiate a glow discharge within a gas-filled lamp, such as a conventional flourescent lamp, and thereafter maintain a stable supply of power to the lamp to sustain the discharge. As is well known, conventional electronic ballast systems typically include an inverter circuit that supplies alternating current (AC) power to the lamp and a lamp driver circuit, which uses a pulse-width modulated (PWM) control signal to vary the amount of power that the inverter supplies to the lamp.
As is also well known, the inverter circuit typically includes a power switch (e.g., a transistor) that is switched on and off at a frequency determined by the resonance of a timing capacitor and an inductor. In practice, the capacitance of the timing capacitor may deviate about five to ten percent from an ideal value. As a result, the frequency and duty cycle of the PWM control signal may also vary in proportion to the deviation of the capacitance value from the ideal value, thereby changing the amount of power which is delivered to the lamp. Additionally, the variation in the frequency and duty cycle of the PWM signal prevents precise zero voltage switching control of the power switch, which increases the operating temperature of the power switch and significantly reduces its expected operating life.